1. Field of the Invention
The present invention relates generally to arc welding control systems, and relates more particularly to adaptive welding systems which sense the characteristics of the seam being welded in order to adjust the center line of tracking of the welding electrode in the seam being welded, the width of oscillation, and the wire feed rate and/or travel speed for constant fill control.
2. Description of the Prior Art
In many arc welding applications, it is desirable to employ a system which guides an arc welding electrode along a joint or seam in a workpiece which is being welded. Such applications require the use of some type of weld joint tracking system, which generally senses the position of the welding electrode relative to the joint being welded and guides the electrode along the seam during the welding process. For example, tracking systems have been constructed which employ electromechanical probes within the joint, optical sensing devices which detect light emitted by a light source and reflected from the weld seam, and devices which detect a voltage or current signal characteristic of the distance between the arc electrode and the sidewall of the joint.
Each of the known methods for joint tracking has proven satisfactory for certain applications, but each present drawbacks depending on factors such as the constancy of the weld joint geometry, the unsuitability of employing either contacting or non-contacting transducers for sensing position, and the high control circuit noise levels produced by the welding power supplies.
The particular type of arc welding employed has a significant impact on noise levels which must be overcome, especially when weld tracking systems utilize the voltage or current signal measured between the workpiece and the weld electrode. Since the vertical electrode-to-workpiece spacings are known to be proportionally related to the arc voltage or current signal (depending upon the welding process), accurate measurements of the arc signal are required in order to provide reliable control. In gas tungsten arc welding (GTAW), the control circuitry senses the welding voltage which varies as a function of the distance between a nonconsumable tungsten arc electrode and the workpiece. Metal is deposited into the arc by a feed wire. In gas metal arc welding (GMAW), a consumable metal electrode is employed to deposit metal in the weld joint, and the arc current is sensed in order to derive tracking information. Submerged arc welding (SAW), plasma arc welding (PAW), and flux core arc welding (FCAW) are other arc welding systems in which tracking information may be derived from the voltage or current of the welding arc.
Of the above different types of arc welding systems, GTAW provides the "cleanest" signal because the electrode is always maintained a distance above the workpiece and never directly contacts the workpiece. GMAW, on the other hand, presents weld current waveforms which are often significantly noisier than the weld voltage waveforms encountered in GTAW. In GMAW, there are three different modes of operation, each of which produce different levels of noise in the current signal. In the spray transfer mode, the metal deposited leaves the consumable electrode in a fine mist of droplets which is directed to the workpiece in a fairly well-defined column and generally presents a signal waveform about as stable as the waveform in the GTAW process. In the dip transfer or short arc mode of GMAW, the electrode actually touches the workpiece on the order of 20-200 times per second and produces a short-circuit in the welding circuit. The frequency of short-circuiting is directly related to noise level, and varies as a function of the gas fed into the arc and capabilities of the power supply to provide current surges when the short-circuit occurs. When the short-circuit frequency reaches as low as 50 Hz, conventional noise removal circuitry invariably slows down the tracking process. In the globular transfer mode of GMAW, large globules of metal are deposited from the electrode to the workpiece at a frequency of between 10-20 Hz, but the frequency of short-circuiting varies widely and is very erratic. Consequently, it is even more difficult to acquire useful position information and still maintain an acceptable speed of tracking and welding.
Added to the noise level complication is the requirement that acceptable quality welds be produced. It is known to oscillate the electrode laterally across the groove in order to distribute metal across the seam as the electrode is moved along the path of the seam. Generally, the electrode must dwell for a period of time at the extremes of oscillation in order to ensure good weld tie-in at the side walls of the groove.
Some welding oscillation control systems remove noise by sampling the arc signal a plurality of times as the electrode dwells at the extremes of oscillation. For example, in the application entitled "Weld Tracking/Electronic Arc Sensing System", Ser. No. 054,517, filed July 3, 1979 by George E. Cook et al. now U.S. Pat. No. 4,336,440, there is disclosed a weld tracking system which senses the position of the welding electrode when the electrode has reached a predetermined lateral distance from the bottom of the weld groove (or "null") and is located at a given position relative to the sidewall of the weld groove. The arc signal is sampled a number of times at the lateral extension of oscillation while the electrode dwells at the extremes of movement. The samples are averaged, and the averages at each extreme are compared and a difference between the averages is calculated and used to re-establish the null position so as to minimize the difference in averages. The electrode is maintained at its predetermined distance by an automatic arc voltage control system. Normally, this automatic voltage control system is inactivated during oscillation to avoid cancelling the readings taken at the lateral extremes of oscillation.
The foregoing application and other prior art welding systems have not appreciated the need, in GMAW welding processes, to avoid excessively long dwell times which slow the welding. With the GTAW process, however, the information regarding position can be obtained in much less time because the signal bears less noise. Typically, 16 milliseconds is adequate time to dwell at the sidewalls in GTAW welding. In most GMAW welding, however, data should be obtained over several short-circuiting cycles in order to obtain a signal reliably indicative of the spacing between the arc electrode and the sidewall of the workpiece. For example, if the shorting frequency is of the order of 100 Hz (10 milliseconds per shorting cycle), data samples should be taken over approximately 5 cycles which requires about 50 milliseconds. If the shorting frequency drops to 25 Hz, or about 40 milliseconds per cycle, then taking data samples and averaging over four or five short-circuit cycles means dwelling for 160-200 milliseconds at the extremes of oscillation while the samples are taken and the average is computed.
It has been determined that a fixed period for sampling of between 100-200 milliseconds is generally sufficient to assure reliable data for short arc and other GMAW processes. For high speed welding, the welding apparatus cannot afford to dwell at the extremes of oscillation on the sidewalls for extended period of times to take samples and still maintain high welding speeds. At welding speeds of about 30 inches per minute, a period of dwell at the extremes on the sidewalls of 150 milliseconds is reasonable and allows a sufficient number of data samples to be taken. However, if welding speeds as great as 60 inches per minute are desired, the dwell time is effectively halved to about 75 milliseconds, which precludes taking a sufficient number of data samples to insure reliability. Consequently, the apparatus disclosed in the foregoing application is unable to attain high welding speeds due to its inherent inability to obtain sufficient data samples while the electrode is dwelling at the outermost extremes of oscillation.
In addition to dwell time problems, filtering of the arc signal slows down the ability of prior art systems to rapidly and reliably track. A low pass analog filter is used to filter undesirable noise in some prior art weld tracking equipment. Additional time is required for the signal to propagate through the analog filters prior to sampling. There must therefore be a coordination between the sampling and the output of the filter in order to accomplish the signal processing within a practical time limit. When the dwell time is added to the signal filter propagation delay time, there is a great risk that the conventional analog-filter apparatus cannot reliably track in GMAW at satisfactory speeds. Thus, unless alternative approaches are taken for GMAW processes, an automatic control system employing data sampling at the extreme positions of oscillation will result in unacceptably slow tracking speeds.
Prior art systems have generally failed to address problems which arise in GMAW processes because there is no time to take sufficient data samples at the extremes of oscillation to provide meaningful position information. If prior art systems are adapted to allow the taking of additional data at the extremes of oscillation, the quality of the weld will suffer because the electrode lingers for too long a period at the extremes of oscillation prior to departure.
It is now believed that there is a distinct advantage in taking data samples during the lateral movement of the electrode across the joint, due to the tendency to melt down the sidewall if the electrode dwells for an excessively long period of time. Prior tracking apparatus in which samples are taken only at the extremities of oscillation, does not utilize the time for deceleration and settling and acceleration of the electrode to obtain data samples. For all intents and purposes, the electrode resides near the extremes of oscillation during the periods of acceleration, settling, and deceleration, and additional data samples could conceivably be taken with the data samples taken while the electrode is settled in order to derive reliable position information.
The apparatus disclosed in the aforementioned application is unable to obtain data samples at any point in the oscillation cycle other than at the extreme lateral points of oscillation, because the microcomputer is busy controlling the lateral movement whenever the electrode is in motion. Only when the electrode has settled at the lateral extreme of oscillation can the microcomputer take data samples and perform the computations required to insure that a reliable position signal is being received.
The British published patent specification No. 1,517,481 of Takagi and Nishida also discloses a welding control apparatus which obtains position information only at or near the vicinity of the extremes of electrode movement. In one embodiment, the position signal is summed over a variable number n oscillations or weaving cycles, but it appears that a sample is taken of the signal only once per oscillation. In another embodiment, the position signal is integrated for a predetermined time period and then summed over n weaving cycles. However, the samples and integrations of the samples appear to occur only at the extreme ends of the oscillations. The overall welding speed is thus slowed due to this need to accumulate data over several oscillations in order to ensure reliable position information.
The patent to Kushner, U.S. Pat. No. 4,151,395, discloses another approach to controlling the lateral position of the electrode. A signal representative of the position of the arc electrode as it travels the weld path between opposite sides of the workpiece is first obtained by processing through a noise filter. The signal is then combined with a timing signal generated at the extremes of oscillation of the electrode to obtain a peak value indicative of the extreme lateral position of the electrode as it moves from side to side. These peak values are compared with each other or with other signals of a known value to obtain position control signals, such as the width of oscillation of the electrode or the correction of the centerline of travel of the electrode along the weld joint. This apparatus, however, also obtains arc signals only at the extremes of oscillation, and thus cannot rapidly and reliably track for GMAW processes.
The patent to Smith, U.S. Pat. No. 3,646,309, discloses a different approach to maintaining the welding electrode within the weld groove. The Smith apparatus includes an inhibit signal generator which is responsive to a preset arc voltage to generate an inhibit signal to stop the lateral oscillator drive when the electrode approaches too closely to the sidewalls of the weld groove. The inhibit condition is maintained until the oscillator drive is reversed to begin oscillation of the electrode toward the opposite sidewall. Although Smith maintains the electrode within a predetermined width of oscillation, the Smith device cannot respond to increases in the width of the weld groove by increasing the amount of lateral movement. Lateral movement is inhibited only if a preset value is reached prior to the arc electrode reaching the lateral extreme of movement. Moreover, the Smith apparatus would be unsuitable in GMAW processes because no means are disclosed for insuring that spurious noise signals do not provide false indications that the preset value has been reached. The Smith apparatus would therefore be generally unsuitable for use in GMAW processes because of a lack of noise filtering and signal processing capability to insure that meaningful position information has been acquired before action is taken.